It is known both in photographic elements for conventional photographic processing and diffusion transfer photographic processing to provide a temporary barrier layer among layers into which processing compositions permeate. Particularly in diffusion transfer photographic processing, a neutralizing layer is employed for purposes such as stopping development, stabilizing images, etc. It is well known to use a temporary barrier layer for purposes of "timing" neutralization, i.e., to prevent reduction of the maximum density due to premature neutralization of a developing solution by this neutralizing layer. The temporary barrier layer used for such purposes is called a timing layer and is disclosed in, for example, U.S. Pat. Nos. 4,061,496, 4,056,394 and 4,201,587; Japanese Patent Application (OPI) Nos. 72,622/78 and 141644/82 (corresponding to U.S. Pat. No. 4,199,362) (as used herein the term "OPI" refers to a "published, unexamined Japanese Patent Application").
In silver halide photography development is generally slow at low temperatures and rapid at high temperatures. In the case of instant photography employing diffusion transfer processing, development is conducted at various temperatures, and not at controlled temperatures as in conventional photography. In order to obtain good images even at varied development temperatures, it is extremely important for photographic elements to be able to compensate for temperature in neutralization of a processing solution, such that neutralization is rapidly completed at high temperatures because development rapidly proceeds at high temperatures, but, that neutralization is prolonged at low temperatures because development proceeds slowly. In the patents and patent applications cited above, many timing layers are disclosed which have the function of compensating for temperature.
Conventional mono-sheet diffusion transfer photography processes have the disadvantage that density of the image generally increases, in maximum density areas, gradation areas and minimum density areas, to seriously decrease image quality with the passage of time (from several days to several weeks after the formation of images). This phenomenon is referred to as "post-transfer", and occurs because the dyes released (liberated) during processing do not all immediately move to a mordanting layer, but partially remain in layers other than the mordanting layer. With the passage of time, these remaining dyes are gradually transferred into the mordanting layer. Cover sheets having timing layers as disclosed in the patents and patent applications cited do not have any function of preventing such post-transfer.
U.S. Pat. No. 4,356,249 teaches that a first timing layer and a second timing layer may be provided as timing layers (provided that the temperature coefficient is negative for the first timing layer), along with a conventional neutralizing layer, and an auxiliary neutralizing layer is provided between the first timing layer and the second timing layer to thereby improve processing temperature dependency of a color diffusion transfer photographic film unit (assemblage) containing a positive type redox compound as a dye image forming compound. In this photographic system which improves processing temperature dependency, the auxiliary neutralizing layer is designed so as to control the degree of release of a dye from a positive type redox compound caused by development of silver halide at low temperatures on a markedly higher level than that at high temperatures. However, this photographic system does not present post-transfer, since the system does not intend to prevent post-transfer of the diffusible dye released from the positive type redox compound.